ax25: uninitialized variable in ax25_setsockopt()

The "opt" variable is unsigned long but we only copy 4 bytes from
the user so the lower 4 bytes are uninitialized.

I have changed the integer overflow checks from ULONG to UINT as well.
This is a slight API change but I don't expect it to break anything.

Fixes: a7b75c5a ("net: pass a sockptr_t into ->setsockopt")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>